Resistance heating element



Feb. 21, 1967 T. H. LENNOX RESISTANCE HEATING ELEMENT Filed March 23, 1964 11/04 145 ELEM/vex, Ivre-A/rae.

6% wmwggwm firmed/EM United States Patent 3,305,820 RESISTANCE HEATING ELEMENT Thomas H. Lennox, 1947 Via Santiago,

Corona, Calif. 91720 Filed Mar. 23, 1964, Ser. No. 354,070 4 Claims. (Cl. 338240) This invention relates to high temperature electrical resistance heaters and more particularly to an improved form of the type embodying an elongated tubular metallic casing or sheath containing a resistance heating element embedded in a high temperature resistant and heat conducting material which is electrically nonconductive such as highly compacted MgO.

In general, such devices comprise a metal tube having a longitudinally extending helical coil disposed in the axial line thereof and being secured in place by filling the tube with powdered MgO and then subjecting the tube and contents to a swaging operation to reduce the diameter of the tube sufficiently to compact the MgO into rock-like hardness. As demands for higher and higher temperatures from heating elements of this type have been made, different high temperature alloys have been employed for the tubes, such alloys usually including a chromium-nickel content. However, at increasing tem peratures ranging upwardly from 1600 F. the heaters shorted out much sooner than was expected. Investigation disclosed that this decreased life of heaters having casings of improved heat resisting alloys was an unexpected by-product of the normal breathing of the heater. In heaters of this general character, some air remains entrapped in the compacted mass of MgO and when the heater is energized, the expansion of the entrapped air causes a portion of it to be expelled through the terminal end of the heater casing. As the heater cools as, for example, when it is temporarily deactived by a thermo-responsive control means, the contraction of the heated air remaining in the MgO mass creates a partial vacuum in the casing due to the fact that the atmospheric pressure is much less than the pressure which caused the expulsion of air. This expulsion and re-entry of air in heaters of this character is what is known as breathing.

Recurring now to the present invention and the investigation from which it derived, it was found that when current was supplied sufficient to cause a heat output which theoretically could be withstood by the improved alloys employed for the casing, the chromium content of the casing alloy tended to become vaporized and invade the MgO mass. This is due to the fact that the boiling point of the chromium content of the alloy is lower than the boiling point of iron and nickel, the other constituents of the casing alloy. This, of course, results in the eventual destruction of the nonconductive character thereof and consequent shorting out of the heater. Further investigation disclosed that this invasion of the MgO mass by the chromium was the result of the vacuum deriving from the breathing action of the heater, it being evident that the higher the heat, the greater the vacuum with resultant acceleration of the vaporization of the chromium and consequent invasion of the MgO mass. I

In addition to the above described unexpected deleterious action, this invasion of the MgO mass by the chromium had a second deleterious result in that, by in effect, boiling out of the casing alloy, the structural integrity of the alloy of the unprotected casing was adversely affected and the heat at which it would maintain rigidity was progressively lowered as the chromium content was thus dissipated. So far as efforts to increase the heating capacity of heating devices of this character has been directed to the creation of heat resistant alloys 3,305,820 Patented Feb. 21, 1967 for the casing in which the chromium content was somehow held in the alloy by reason of the structure or composition of the alloy itself at atmospheric pressure.

The present invention bypasses all of these attempts to improve the alloy of the casing and is directed to the preservation of the content and the structural integrity of the casing means by interposing a wall or layer of mild steel between the chromium containing sheath or casing and the body of compacted MgO since the negative pressures developed in the MgO mass would not be transmitted to the chromium and any chromium leaving the alloy would be absorbed by the said mild steel layer thus interposed between the chromium containing outer sheath and the MgO mass. Under these conditions, since the chromium remains out of the MgO mass, said mass is able to maintain operative electrical dielectric strength at temperatures at which it would otherwise become unstable, wherefore the upper range of heat derivable from the heater can be significantly increased. Further, the oxidation of the mild steel liner, especially under conditions of electrical cycling is prevented with attendant breathing of the device by a slight buildup of a scale on the steel that does not migrate into the MgO. This is due to the fact that the coefiicients of thermal expansion of the steel and the MgO are very close.

Accordingly, the principal object of the invention is to provide an electrical resistance heating device for high temperatures including an elongated tubular external casing of a chromium-nickel alloy for structural integrity and including means for maintaining the content integrity of the casing alloy under conditions of heat which otherwise would cause it to destroy the stability of the electrical insulation.

A further object of the invention is to provide a device of the above character in which the foregoing objective is realized in practice, which is simple in construction, and which can be manufactured with the apparatus and manufacturing facilities ordinarily employed for heating elements of that general character.

With the foregoing objects in view, together with such additional objects and advantages as may subsequently appear, the invention resides in the parts and in the construction, combination and arrangement of parts described, by way of example, in the following specification of certain presently preferred embodiments of the invention, reference being had to the accompanying drawings which form a part of said specification and in which drawings:

FIG. 1 is a side perspective view of a heating device constituting a first embodiment of the invention,

FIG. 2 is an enlarged scale fragmentary medial sectional view showing the interior of the device shown in FIG. 1, a portion of the midlength of the device being broken out to permit showing on the enlarged scale,

FIG. 3 is the further enlarged scale fragmentary view, partly in section, showing a second embodiment of the invention, and

FIGS. 4 and 5 are transverse sectional views taken on the lines 4-4 and 5-5, respectively, on either FIGS. 2 or 3.

Referring to the drawings and particularly FIGS. 1, 2, 4 and 5, the illustrated first embodiment of the invention comprises a tubular metal outer sheath 1 formed of a chromium-nickel containing, high heat resisting alloy and having an end closure member 2 of the same material closing one end thereof. Disposed Within the external sheath and closely embraced thereby is the tubular internal sheath element 3 formed of mild steel and having one end thereof closed by an end closure member 4 also of mild steel disposed in juxtaposition to the end member 2 of the outer sheath 1. Disposed concentrical- The end member 7 which comprises an electrically conductive metallic means that is positioned within the tube carries an axially disposed metal pin 8 to which one end of a helical resistance wire element 9 is conductively connected, the other end of said resistance element being connected to a metal terminal rod 10 axially disposed within the other or open end of the tube 5. A mass 11 of similarly compacted MgO within the tube 5 serves to position the resistance element 9 and the terminal rod 10 in the axial line of the tube 5. r

The tube 5 projects beyond the open ends of the sheath components 1 and 3 and carries a set of mica washers 12 bearing against the open ends of the sheath components and held thereagainst by a terminal member 13 welded to and projecting tangentially from the tube 5, a metal washer 14 being interposed between the terminal member 13 and the outermost one of the mica washers 12.

The terminal rod 10 projects beyond the end of the tube 5 and carries a set of mica washers 15 which close the end of the tube 5; said washers 15 being held thereon by the end of a terminal sleeve 16 welded or brazed to the exposed end of the rod 10. The terminal sleeve 16 is provided with .an externally threaded, reduced diameter outer end 17 carrying a nut 18 adapted ot clamp one end of the lead L1 against the shoulder 19 formed by the juncture of the said threaded end with the body of the sleeve 16. The terminal member 13 carries a screw 20 threaded into it operative to clamp the end of a lead L2 thereto.

' The electrical circuit thus extends from the terminal member 13 through the tube 5 and end member 7, the rod 8, resistance element 9, terminal rod 10 and terminal sleeve 16. As the device reaches a temperature at which it is incandescent and at which the chromium content of the outer sheath 1 is sufiiciently fluid to otherwise move out of the sheath material, the mild steel tubes 3 and 5 prevent the boiling out of the chromium both from the sheath 1 and the resistance element 9 and thus prevent the shorting out of the device through the contamination of the MgO mass. This, of course, also preserves the content integrity and the resultant rigidity of the sheath 1.

Referring to FIG. 3, there is shown a modified form in which a more complete seal for the exposed end of the MgO mass surrounding the resistance means is provided. Since all other parts are the same as in the first described embodiment, the same numbers have been applied and the description thereof need not be repeated.

In this embodiment of the invention, the washers 15 at the end of the tube 5 are replaced by a ceramic sleeve 21 of greater diameter than the tube 5 which sleeve is mounted on the terminal rod 10', said rod being some: what longer than the terminal rod 10 of the first embodiment. A metal sleeve 22 has a reduced diameter end 23 surrounding and soldered or brazed to the exposed end of the tube 5 outwardly from the terminal 13 and is provided with a skirt portion 24 fitting over and brazed or other secured to the adjacent end of the sleeve 21. Mica washers 25 are carried by the interior of the sleeve 22 and are interposed between the adjacent ends of the ceramic sleeve 21 and the end of the tube 5.

The outer end of this ceramic sleeve 21 carries the skirt portion 26 of a cylindrical binding post sleeve 27 having a reduced diameter externally threaded end 28 which is brazed to and surroundsv the outer endof the terminal rod 10'; said threaded end carrying the nut 18 to clamp the terminal L1 in the same manner as in the first embodiment. This structure forms a more complete seal for the terminal end of the device to resist the entry of air into the terminal end of the device with resultant decrease in the rate of oxidation of the metal surfaces within that end.

The result of thus preventing the migration of the chromium from the external sheath is that the device can be employed without loss of structural or electrical conductive integrityto produce heat on the order of several hundred degrees higher than would otherwise be possible.

It will be recalled that the tube 5, like the tube 3, is formed from mild steel. This has several important, results. First, the mass 6 of MgO between these two tubes is thus confined between surfaces having identical coefficients of linear thermal expansion, wherefore, no stresses deriving from thermal expansion are created by reason of different coefiicients. Also, the mass 11 of MgO within the tube 5 is likewise surrounded by a surface having a uniform coeflicient of linear thermal expansion so that no longitudinal stresses deriving from thermal changes are created withinor outside of the tube 5. Further, while there is a slight difference in the rates of linear thermal expansion between the compacted MgO and the mild steel, at the operating temperatures the tube 5 lacks any structural strength and is free to compensate for such slight difference as might occur without imposing any stresseson the heater.

While in the foregoing specification there have been disclosed certain presently preferred embodiments of the invention, the invention is not to be deemed to be limited to the specific details of construction thus disclosed by way of example, and will be understood that the invention includes as well all such changes and modifications in the parts and in the construction, combination and arrangement of parts as shall come Within the purview of the appended claims.

I claim:

1. A resistance heating element for operation at temperatures in excess of 1600 F. comprising an elongated, first tubular metal outer sheath means comprising a tubular inner component formed of steel having no chromium content and a tubular outer component formed of a chromium containing alloy coextensive with and tightly embracing said inner component, said first outer sheath means including front and rear end portions, said alloy being capable of remaining structurally stable at temperatures at which said inner component loses structural stability, the front end portions of said tubular inner and outer components each being provided with end closure members of a material compatible with the inner and outer components, respectively, a second elongated tubular metal inner sheath means disposed in spaced coaxial relation within said outer sheath means including front and rear ends, electrically conductive metallic means forming a closure for said front end of said second inner sheath means, an electrical resistance assembly disposed in spaced coaxial relation within said second inner sheath means, one end of said electrical resistance being connected to said closure means, a first mass of compacted magnesium oxide disposed between and holding said outer and inner sheath means in spaced parallel relation, a second mass of compacted magnesium oxide disposed in and filling said inner sheath means and holding said resistance assembly in said spaced coaxial relation therein, the rear end portion of said first outer sheath means and said inner sheath means being provided with insulating closing means to prevent the entrance of atmosphere into said first mass and said second mass of magnesium oxide, and terminal means for completing an electrical circuit through said electrical resistance assembly and inner sheath means. I

A r s t nce heating element as claimed in claim 1 in which said second inner sheath means projects beyond an end of said first outer sheath means and is provided With a terminal for connection in an electrical circuit, in which the end of said electrical resistance assembly remote from said one end of said second inner sheath means is electrically connected to said second inner sheath means and in which the other end of said electrical resistance assembly is disposed beyond said projecting end of said second inner sheath means and is provided with a terminal for connection in an electrical circuit to complete a circuit through said resistance assembly.

3. A resistance heating element as claimed in claim 1 in which one end of said electrical resistance assembly extends beyond the adjacent end of said second inner sheath means and is provided with means operative to resist the entrance of atmosphere into said second mass of magnesium oxide.

4. A resistance heating element as claimed in claim 6 1 in which said second inner sheath means and at least the inner surface of said outer sheath means are formed of metal having the same coefiicient of linear thermal ex pansion.

References Cited by the Examiner UNITED STATES PATENTS 2,036,788 4/1936 Abbott 338238 2,063,642 12/1936 Vanden Berg 338238 X 2,703,355 3/1955 Hagglund 338-238 X 2,747,074 5/1956 Finch 338-238 X 2,767,288 10/1956 Lennox 338-238 FOREIGN PATENTS 994,986 6/1965 Great Britain.

RICHARD M. WOOD, Primaly Examiner.

V. Y. MAYEWSKY, Assistant Examiner. 

1. A RESISTANCE HEATING ELEMENT FOR OPERATION AT TEMPERATURES IN EXCESS OF 1600*F. COMPRISING AN ELONGATED, FIRST TUBULAR METAL OUTER SHEATH MEANS COMPRISING A TUBULAR INNER COMPONENT FORMED OF STEEL HAVING NO CHROMIUM CONTENT AND A TUBULAR OUTER COMPONENT FORMED OF A CHROMIUM CONTAINING ALLOY COEXTENSIVE WITH AND TIGHTLY EMBRACING SAID INNER COMPONENT, SAID FIRST OUTER SHEATH MEANS INCLUDING FRONT AND REAR END PORTIONS, SAID ALLOY BEING CAPABLE OF REMAINING STRUCTURALLY STABLE AT TEMPERATURES AT WHICH SAID INNER COMPONENT LOSES STRUCTURAL STABILITY, THE FRONT END PORTIONS OF SAID TUBULAR INNER AND OUTER COMPONENTS EACH BEING PROVIDED WITH END CLOSURE MEMBER OF A MATERIAL COMPATIBLE WITH THE INNER AND OUTER COMPONENTS, RESPECTIVELY, A SECOND ELONGATED TUBULAR METAL INNER SHEATH MEANS DISPOSED IN SPACED COAXIAL RELATION WITHIN SAID OUTER SHEATH MEANS INCLUDING FRONT AND REAR ENDS, ELECTRICALLY CONDUCTIVE METALLIC MEANS FORMING A CLOSURE FOR SAID FRONT END OF SAID SECOND INNER SHEATH MEANS, AN ELECTRICAL RESISTANCE ASSEMBLY DISPOSED IN SPACED COAXIAL RELATION WITHIN SAID SECOND INNER SHEATH MEANS, ONE END OF SAID ELECTRICAL RESISTANCE BEING CONNECTED TO SAID CLOSURE MEANS, A FIRST MASS OF COMPACTED MAGNESIUM OXIDE DISPOSED BETWEEN AND HOLDING SAID OUTER AND INNER SHEATH MEANS IN SPACED PARALLEL RELATION, A SECOND MASS OF COMPACTED MAGNESIUM OXIDE DISPOSED IN AND FILLING SAID INNER SHEATH MEANS AND HOLDING SAID RESISTANCE ASSEMBLY IN SAID SPACED COAXIAL RELATION THEREIN, THE REAR END PORTION OF SAID FIRST OUTER SHEATH MEANS AND SAID INNER SHEATH MEANS BEING PROVIDED WITH INSULATING CLOSING MEANS TO PREVENT THE ENTRANCE OF ATMOSPHERE INTO SAID FIRST MASS AND SAID SECOND MASS OF MAGNESIUM OXIDE, AND TERMINAL MEANS FOR COMPLETING AN ELECTRICAL CIRCUIT THROUGH SAID ELECTRICAL RESISTANCE ASSEMBLY AND INNER SHEATH MEANS. 